Researchers with the Centers for Disease Control and Prevention determined a whopping 90 percent of the about 45,000 HIV transmissions that occurred in 2009 were either undiagnosed or not yet receiving medical care.

The findings suggest more must be done to treat and prevent HIV throughout the entire progression of the illness in patients, the JAMA Network Journals reported. Interventions at each phase of the care continuum (diagnosis, retention in medical care, prescription of antiretroviral therapy [ART] and viral suppression) could help reduce HIV transmissions. Estimating the number and rate of HIV infections that occur during each step of the care continuum could help policy makers make better use of their HIV prevention resources.  

"In the United States, persons living with HIV who are retained in medical care and have achieved viral suppression are 94 percent less likely to transmit HIV than HIV-infected undiagnosed persons. Unfortunately, too few persons living with HIV have achieved viral suppression. These estimates of the relative number of transmissions from persons along the HIV care continuum highlight the community-wide prevention benefits of expanding HIV diagnosis and treatment in the United States," the researchers wrote in the study.

The researchers looked at national databases to estimate the rates of HIV transmission seen in the U.S. in 2009. The data showed there were more than 1.1 million people living with HIV, out of these individuals: 18.1 percent were undiagnosed; 45.2 percent knew of their infections but were not in medical care; 4.1 were receiving medical care but were not prescribed ART; 7.2 percent were prescribed ART but were not virally suppressed; and 25.3 percent had achieved viral suppression.

When compared with individuals who were infected with HIV but undiagnosed , those who were diagnosed but not in medical care were 19 percent less likely to transmit the virus; individuals who were virally suppressed were 94 percent less likely to transmit HIV.

"Improvements are needed at each step of the continuum to reduce HIV transmission. Through stronger coordination of efforts among individuals, HIV care providers, health departments and government agencies, the United States can realize meaningful gains in the number of persons living with HIV who are aware of their status, linked to and retained in care, receiving ART, and adherent to treatment," the study concluded.

The findings were published in a recent edition of the journal JAMA Internal Medicine.